Superplastic alloy of tin and zinc

ABSTRACT

A superplastic alloy of tin and zinc is disclosed of nominally eutectic composition (91 percent by weight of tin and 9 percent by weight of zinc) with an actual analysis of 9.8 percent zinc in tin, the alloy having an average grain size ranging between 1-2 microns (0.001-0.002 mm) and exhibiting superplastic behavior at temperatures of approximately 250*F. and higher with a maximum index of strain rate sensitivity of 0.4 to 0.5. The alloy shows elongations under tension of 570 percent at temperatures between 300*F.-350*F.

United States Patent [1 1 Prematta et a1.

1 1 SUPERPLASTIC ALLOY OF TIN AND ZINC [75] inventors: Robert Joseph Prematta,

Hightstown; Peruvemba Swaminatha Venkatesan, Princeton, both of NJ.

[73] Assignee: Western Electric Co., Inc., New

York, NY.

[22] Filed: Apr. 25, 1974 [21] Appl. No.: 464,010

[52] US. Cl l48/l1.5 R; 75/175; 148/32 [51] Int. Cl. C22F 1/16 [58] Field of Search 75/175, 178 T, 135;

[56] References Cited UNITED STATES PATENTS 8/1973 Swanson 148/1 1.5 R 10/1974 Cross et al. v. 148/115 R OTHER PUBLICATIONS Hansen, Constitution of Binary Alloys,"

[4 1 Dec. 9, 1975 McGrawHill, 1958, pp. 1217-1219.

Primary Examiner L. Dewayne Rutledge Assistant Examiner-E. L. Weise Attorney, Agent, or Firm-D. P. Kelley; A. S. Rosen; R. Spencer [57] ABSTRACT 5 Claims, 2 Drawing Figures US. Patent Dec. 9, 1975 Sheet 1 of2 3,925,110

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(4 SSEIHLS SUPERPLASTIC ALLOY OF TIN AND ZINC BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates broadly to superplastic alloys. More specifically, this invention relates to a superplas tic alloy of tin and zinc.

2. Description of the Prior Art Superplastic alloys constitute a fairly newly discovered class of metallic materials which is characterized by neck-free elongations phenomenally exceeding the elongations of ordinary metallic materials.

Ordinary metallic materials cannot normally be stretched by more than l percent no matter what the temperature is or how slowly tension is applied. Thus, a typical elongation for an alloy steel is approximately l0l5 percent, and most aluminum alloys are considered quite plastic if their elongations reach 50 percent. A superplastic alloy, on the other hand, may be elongated as much as 2000 percent without rupture.

The relationship between the steady-state value of stress during plastic deformation (i.e., the level of stress at which the stress-strain curve has bent over and runs essentially parallel to the strain axis), 0', and the strain rate, 6, associated with that value of stress can be expressed by the equation:

a x" (Eq. I where x is a constant and m is the index of strain rate sensitivity. When m is large, the family of stress-strain curves associated with any material (each of said stressstrain curves being identified with some particular rate of strain) is widely spaced, i.e., the behavior of the material is very much dependent upon the rate at which tension is applied. When m is small, the family of stressstrain curves is closely spaced, i.e., the behavior of the material is essentially independent of strain rate.

In normal (i.e., non-superplastic) metallic materials, m is generally small, usually less than 0.2. In superplastic metallic materials, m is large, ranging from about 0.2 to about 0.8. In physical terms, this means that the behavior of a superplastic tensile specimen depends a great deal on how rapidly tension is applied. This is one criterion of superplasticity.

A high sensitivity to strain rate (i.e., a large value of m) is responsible for the enormous elongations characteristic of superplastic metallic materials. Thus, differentiating Equation I yields:

Neglecting the second term on the right, it will be seen that the larger the .value of m, the less effect any change in stress will have on a change in strain rate, thus implying quite unusual behavior of superplastic metallic materials. When a specimen of a normal metallic material is subjected to tension, it begins to stretch uniformly, but eventually some local instability causes the specimen to neck down." This is normally the beginning of the end as the stress at the neck," by virtue of the reduced cross section, is larger than the stress in the rest of the specimen. As a result, most of the elongation is concentrated at the neck and that is where the specimen eventually ruptures. In a superplastic metallic material, the large value of m greatly reduces the tendency for this to happen as a change in dd l stress such as occurs at the neck, does not cause a very large change in the strain rate, and the area around the neck deforms at a rate comparable to deformation in the remainder of the specimen.

To continue this discussion, m, in a more specific sense, is not a constant but actually varies with strain rate. In normal metallic materials, m usually decreases slightly with strain rate. In superplastic metallic materials, m increases with strain rate up to a certain level and decreases thereafter. In normal metallic materials, such negative variation in m will cause the last term on the right in Equation 2 to be added to the preceding term, thus making sensitivity to necking down even more pronounced. Positive variation in m in superplastic metallic materials helps to reduce sensitivity to necking down. In fact, in certain situations, the positive variation in m may be large enough so that the term approaches zero, thereby essentially eliminating the tendency of material to neck.

One of the distinguishing characteristics of superplastic metallic materials is their extremely small grain size, of the order of 0.001 mm., the grain size of normal metallic materials ranging between 0.] mm. and several millimeters; Thus, the transformation of a superplastifiable metallic material to its superplastic state may be accomplished, in part, by suitable heat treatment to change the ordinary range of grain size to the extremely small grain size characteristic of superplastic metallic materials. A superplastic metallic material can be transformed to a non-superplastic state by appropriate heat treatment to increase the grain size to the normal range. Another distinguishing characteristic is a twophase or more than two-phase micro-structure.

Another of the distinguishing characteristics of superplastic metallic materials is the presence of a critical temperature below which the metallic material is merely superplastifiable and above which the metallic material is superplastic (provided, of course, that its grain size is extremely small, as hereinabove described). The critical temperature at which superplastic behavior occurs is greater than one-half the melting point of the material under consideration.

Summarizing to this point, the terms superplastic" and superplastifiable may be defined as follows:

1. A superplastic metallic material, broadly speaking, is a metallic material characterized by unusually large elongation under tension with small sensitivity to necking down. More specific ally, a superplastic metallic material is one having:

a. an index of strain rate sensitivity, m, from Equation l, lying between about 0.2 and 0.8; and

b. an extremely small grain size of the order of 0.001

mm. which is stable during deformation; and

c. a temperature at or above the critical temperature (i.e., greater than one-half the melting point).

2. A superplastiflable metallic material has the extremely small grain size characteristic of superplastic metallic materials, but is at a temperature below the critical temperature (i.e., less than one-half the melting point), and can be brought to the superplastic condition by elevating its temperature to or above the critical temperature.

Alloys known to be capable of assuming the superplastic state are listed in the following table. The index of strain rate sensitivity, m, has been given where available.

m, Index of Approximate Superplastic alloys of tin and zinc have not heretofore been known.

SUMMARY OF THE INVENTION One of the objects of this invention is to provide a new superplastic alloy.

A specific object of this invention is to provide a superplastic alloy of tin and zinc.

Other and further objects of this invention will become apparent during the course of the following description and by reference to the appended claims.

Briefly, we have discovered a superplastic alloy of tin and zinc, of nominally eutectic composition, the alloy having an average grain size ranging between 1-2 mlcrons. The alloy exhibits superplastic behavior at temperatures of approximately 250F. and higher.

DESCRIPTION OF THE DRAWINGS FIG. 1 shows plots of stress vs. strain rate for the superplastic alloy of tin and zinc herein described, for temperatures ranging between 250F. and 350F. in increments of 25F".

FIG. 2 shows plots of m, the index of strain rate sensitivity, vs. the strain rate for the superplastic alloy of tin and zinc herein disclosed, for temperatures ranging between 250F. and 350F. in increments of 25F., showing further the attainment of maximum indiees of strain rate sensitivity ranging between approximately 0.4 and 0.5 for these temperatures.

DESCRIPTION OF THE PREFERRED EMBODIMENT Tin and zinc, in nominally eutectic proportion (91 percent by weight of tin and 9 percent by weight of zinc), were melted and cast into cylindrical billets having a diameter of 2% inches. The cylindrical billets were cold forged to approximately Z'A-inch diameter cylinders which, subsequently, were ground to a 2-inch diameter and thereafter extruded at 200F. to produce %-inch diameter rod. The rod composition was analyzed and showed 9.80 percent zinc in tin, which com- 4 position is so close to eutectic that it can be described as near-eutectic. The rod was found to have an average grain size ranging between 1-2 microns (0001-0002 mm) according to the method of Hilliard (Estimating Grain Size by the Intercept Method," Meta! Progress, Vol. [1964] pp. 99-100).

The %-inch diameter rod was then machined into standard tensile test specimens and subjected to tensile tests at temperatures ranging from 250F. to 350F. on an lnstron Tensile Test Machine Model TT-C capable of crosshead speeds as low as 2 X 10 inches/minute. Readings of crosshead speeds, time and load were taken. Tensile tests below 250F. were not made because it is generally known that superplastic behavior manifests itself only at temperatures above half the melting point of the material under consideration. The melting point of the tin-zinc alloy herein disclosed is approximately 388F., and the lowest test temperature 250F., is not, for the purposes hereof, substantially above half the said melting point.

Table I below shows crosshead speeds and resulting flow stresses in the specimens at the various test temperatures.

From data collected during the course of the tensile tests hereinbefore described, plots of stress vs. strain rate for the several test temperatures were developed and are shown in FIG. 1.

From the same data, plots of m, the index of strain rate sensitivity, vs. strain rate for the several test temperatures were developed and are shown in FIG. 2. It will be noted that m, the index of strain rate sensitivity, reaches a maximum value of between approximately 0.4 and 0.5 for the several test temperatures, attaining a maximum value of approximately 0.5 for the test conducted at 300F. at a strain rate of between 10 and 10 inches per inch per minute. These maxima clearly are well above the bottom, 0.2, of the generally acceptable range for indices of strain rate sensitivity for superplastic materials.

The tensile test specimens attained elongations of 570 percent at test temperatures of 300F. and 350F., clearly characterizing the tin-zinc alloy herein disclosed as superplastic.

What is claimed is:

l. A tin-zinc alloy consisting essentially of about 91 percent by weight of tin and about 9 percent by weight of zinc, having an average grain size ranging between 0001-0002 mm.

2. An alloy according to claim 1 characterized by superplastic deformability at temperatures above approximately 250F.

3. An alloy according to claim 2 characterized by an index of strain rate sensitivity ranging between approxi- 6 zinc alloy comprising providing an alloy consisting essentially of about 91 percent by weight of tin and about 9 percent by weight of zinc, cold forging the alloy, and extruding the cold-forged alloy at 200F. to impart an average grain size thereto ranging between approximately 0.00l-0.002 mm.

* i k i t 

1. A TIN-ZINC ALLOY CONSISTING ESSENTIALLY OF ABOUT 91 PERCENT BY WEIGHT OF TIN AND ABOUT 9 PERCENT BY WEIGHT OF ZINC, HAVING AN AVERAGE GRAIN SIZE RANGING BETWEEN 0.001-0.002 MM.
 2. An alloy according to claim 1 characterized by superplastic deformability at temperatures above approximately 250*F.
 3. An alloy according to claim 2 characterized by an index of strain rate sensitivity ranging between approximately 0.4-0.5 at strain rates of between approximately 10 2 -10 3 inches per inch per minute.
 4. An alloy according to claim 1 characterized by elongations under tension of approximately 570 percent at temperatures between approximately 300*F.-350*F.
 5. A process for imparting superplastic deformability at temperatures above approximately 250*F. to a tin-zinc alloy comprising providing an alloy consisting essentially of about 91 percent by weight of tin and about 9 percent by weight of zinc, cold forging the alloy, and extruding the cold-forged alloy at 200*F. to impart an average grain size thereto ranging between approximately 0.001-0.002 mm. 